This invention relates to composite repair devices in general, and in particular to composite repair devices having resistive heating elements and vacuum drawing means.
The increasing use of composite materials, especially in aircraft structure, has been paralleled by advances in techniques for repairing damaged composite structures. In fact, much of the technology for repair of composites is drawn from the composite forming art itself. In general, the resinous materials forming a composite matrix must be heated to effect curing, and pressure must be applied during curing to insure a proper bond between the laminates suspended in the matrix as well as to insure proper conforming of the composite structure to a mold.
Pressure may be applied to a surface by enclosing the article bearing such surface in a non-porous closure or bag capable of being evacuated and sealed. In this manner a uniform pressure of up to one atmosphere may be applied across the entire surface area in contact with the bag. An example of a prior art device employing this technique is found in U.S. Pat. No. 2,331,296, issued to Bendix.
Several methods of applying heat to effect a cure of the composite adhesive have been used, with varying satisfaction, in prior art devices. In Bendix, for example, heat is applied by circulating an externally heated fluid in a chamber surrounding the evacuated bag. U.S. Pat. No. 2,489,643, issued to Hunter, teaches that the rubber or flexible material of the vacuum bag itself may be made electrically conductive in portions and heated by passing an electric current therethrough. The conductive portions are formed of rubber or other suitably flexible materials impregnated with powdered graphite, carbon black, or powdered metal.
A portable repair apparatus which combines a flexible diaphragm with an integrally formed resistive heating element is found in U.S. Pat. No. 3,837,965, issued to Mahon, et al. The diaphragm of the Mahon device includes a flange which forms an integral, vacuum tight seal when placed against the surface to be repaired, and the Mahon heating element comprises a coil embedded in the diaphragm. It is seen, then, that the Mahon apparatus combines various features from composite forming devices to provide a repair apparatus which is portable, capable of applying uniform pressure to a contoured surface, and which includes its own heat source.
A shortcoming of the Mahon device, however, is that the heat loss from the surface being patched is greater at the outer area of the patch than at the center. Furthermore, this center to edge heat loss differential will vary from one repair job to the next according to the thermal properties of the damaged surface, making it extremely difficult to apply a proper uniform curing temperature, for any given surface to be repaired, across the entire area of the patch. This is a severe limitation for patch materials which must be cured within a very narrow temperature range.
A further shortcoming of Mahon and other prior art devices is that the curing cycle must be controlled manually. Typically the heating elements of prior art designs would burn out due to overheating if allowed to remain on for more than ten minutes at a stretch, since the heat thus produced cannot be instantaneously dissipated throughout the system. Thus, an operator was required to stand by the device to periodically shut off the heater until the temperature stabilized.
Furthermore, some patch materials not only require curing at a specific temperature, but in addition must be brought from room temperature (uncured) to curing temperature at a controlled rate. The desired rate of temperature rise is determined by the particular patching material used. For boron-epoxy patches, for example, a rate of temperature rise of 6.degree. to 7.degree. Farenheit per minute is desirable. For newer composite materials, such as graphite-epoxy, even slower rise times, less than 5.degree. F./min., must be obtained.
At best, then, control of the curing cycle for composite repairs is a delicate process, even with proper temperature monitoring equipment.